Stabilized under-drilling apparatus

ABSTRACT

Improved rotary under-drilling apparatus, of the type used for extending and enlarging the bore hole of a well from some point below the surface, includes a pilot drill bit for drilling a small hole in the desired direction of the well, an under-reamer of the offset or assymmetric type, axially spaced from the drill bit, for enlarging the pilot hole to the desired final diameter, and at least one stabilizing means positioned between the reamer and the pilot drill bit. The stabilizing means has an effective diameter of substantially the same size as the hole drilled by the pilot bit but has no cutting surfaces, so that it acts as a journal bearing against the wall of the pilot hole, both to isolate the drill bit from the radial and moment forces generated by the reamer and also to cause the reamer to rotate concentrically with the axis of the pilot hole. Preferably, two stabilizing means are provided, one adjacent to the lower end of the under-reamer and the other spaced between the first guide means and the upper end of the drill bit.

nite Sttes atet 1 Thompson et al.

[ STABILIZED UNDER-DRILLING APPARATUS [75] lnventors: Charles T.Thompson, Dallas; Bobby J. Thompson, Garland, both of Tex.

[73] Assignee: American Coldset Corporation,

Dallas, Tex.

[22] Filed: Mar. 22, 1973 [21] Appl. No.: 344,051

[52] US. Cl. 175/406, 175/325 [51] Int. Cl. E21b 9/22 [58] Field ofSearch 175/406, 399, 53, 325,

[56] References Cited UNITED STATES PATENTS 1,721,004 7/1929 Debose175/325 2,084,421 6/1937 Wright 175/387 X 2,587,231 2/1952 Schierding...175/387 X 2,589,534 3/1952 Buttolph 175/406 X 2,738,167 3/1956 Williams175/330 X 3,231,029 H1966 Winberg 175/53 3,367,430 2/1968 Rowley 175/385Primary E.raminerFrank L. Abbott Assistant E.raminerRich ard E. Favreau5 Attorney, Agent, or Firml(enyon & Kenyon Reill Carr & Chapin Improvedrotary under-drilling apparatus, of the type used for extending andenlarging the bore hole of a well from some point below the surface,includes a pilot drill bit for drilling a small hole in the desireddirection of the well, an under-reamer of the offset or assymmetrictype, axially spaced from the drill bit, for enlarging the pilot hole tothe desired final diameter, and at least one stabilizing meanspositioned between the reamer and the pilot drill bit, The stabilizingmeans has an effective diameter of substantially the same size as thehole drilled by the pilot bit buthas no cutting surfaces, so that itacts as a journal bearing against the wall of the pilot hole, both toisolate the end of the drill bit.

11 Claims, 10 Drawing Figures PATENIEL nan 3I974 sum aor y ll 7 Q T 5 Pm STABILIZED UNDER-DRILLING APPARATUS BACKGROUND 1. Field of theInvention This invention relates to rotary under-drilling means forextending and enlarging the diameter of a portion of a drilled well holeand, more particularly, to drilling and reaming means in which thereamer is of the offset or assymmetric type.

The terms under-drilling and under-reaming refer to methods forextending and simultaneously enlarging a drilled hole or for enlarging aportion of a previously pilot-drilled hole from a point below thesurface. Typically, when deep oil or gas wells are drilled, a depth isreached at which gas pressure, water seepage, or crumbling of the borehole wall threatens collapse of the hole or flooding if deeper drillingis attempted. At this point the drill string must be withdrawn, a lineror casing inserted, and hydraulic cement pumped down the inside of thecasing and upbetween the outside of the casing and the wall of the borehole, where it is allowed to set. Drilling may then continue until theextended hole is again deep enough to require lining.

The inside diameter of each lined section will be smaller than that ofthe section above it because the casing for the lower section mustnecessarily be able to pass inside the casing of the upper section. Inaddition, there must be substantial clearance between the outer wall ofthe casing and the wall of the bore hole to provide a passageway for thehydraulic cement.

If a standard drill just small enough to pass through the upper casingis used to drill the extended hole, the diameter of the new bore holewill be approximately the same size as the inside diameter of thecasing, and a lower casing having substantially smaller outside diameterthan the inside diameter of the upper casing will be needed to obtainthe proper clearance in the extended bore hole.

For example, a well drilled initially to a nominal diameter of 12%inches will take a casing having an inside diameter of 9% inches. if a9% inch drill is used for the extended hole, the casing for the lowersection will have an inside diameter of only 6 inches. On the otherhand, if the extended hole is under-drilled to a nominal l% inchdiameter, a 7 inch ID lower casing can be used.

2. Description of the Prior Art Tools for performing the under-reamingfunction fall into two general classes. The first class includesretractable reamers, which may be either symmetric or assymmetric, andthe second class includes fixed reamers, which are necessarilyassymmetric in order to pass through a casing smaller than theunder-reamed hole. Both types of reamer are usually combined with eithera separate or integral pilot drill bit so that the hole can besimultaneously drilled and under-reamed.

The retractable reamers have blades or other cutting means that arenormally retracted while the drill string is being lowered through thewell casing and are then extended outward to the desired radius of theunderreamed hole after the reamer clears the bottom of the casing.Examples of drill and retractable reamer combinations are given in thefollowing US. Pats.: No. 668,340, issued on Feb. 19, l90l to W. Plotts;No. 1,025,339, issued on May 7, 1912 to F. E. Tucker; No. 1,906,056,issued on Apr. 25, 1933 to F. A. Golish; and

No. 3,051,255, issued on Aug. 28, 1962 to C. L. Deely.

The Tucker and Deely patents provide examples of the symmetric type ofretractable reamer. The Tucker apparatus includes a cylindrical carrierhaving two angled reaming blades pivotally mounted in opposedlongitudinal slots for angular movement between a retracted position andan extended position. A fishtail drill bit is threaded onto the lowerend of the carrier for cutting a pilot hole somewhat larger than thediameter of the lower end of the carrier. The pilot hole is thenenlarged by the reaming blades which, in their extended position,undercut the inside diameter of the well casing.

Deely discloses a four-bladed reamer with a cylindrical body and havinga separate drill bit threaded into its lower end, both bit and reamerbeing of the diamond type. The four blades of the reamer are held inretracted position within spaced longitudinal slots by shear pins as thedrill string is being lowered. The retracted blades of both the Tuckerand Deely reamers are forced outward by axial movement of a taperedinner plug relative to the reamer body due to the weight of the drillstring, once the drill bit engages the bottom of the hole.

The Plotts patent shows an assymmetric type of retractable reamer havinga single pivoted blade that swings out from a retracted to an extendedposition in a manner similar to movement of the twin blades of theTucker device. Whereas the radial and tangential forces acting on thesymmetrically spaced blades of the Tucker and Deely reamers arebalanced, so that the reamers are self-stabilized, these same forcesacting on the singlebladed, assymmetrically arranged reamer of Plottsare not.

Plotts shows in his drawings an elongated blade type pilot drillthreaded onto the lower end of the reamer. Such a drill might providesufficient bearing surface on the edge of the drill blade diametricallyopposed to the reamer blade to counteract the radial force exerted bythe wall of the bore hole, but it has no bearing surface at right anglesto withstand the tangential forces acting on the reaming cutter.Consequently, the Plotts device will tend to rotate eccentrically and todrill a nonconcentric, and possibly crooked, hole.

The Golish patent is for an underreamer that combines a singleretractable and a circumferentially spaced pair of nonretractabletoothed roller cutters in one unit. When the retractable roller isextended for the under-reaming operation, the reamer provides spacedthree-point rolling contact against the sides of the bore hole. Thus,the radial reaction forces of the hole walls against the cutters areinherently balanced, and there are no tangential forces because of therolling contact.

All of the retractable under-reamers described above have a commondrawback, however, in that they have a number of moving parts that canjam or break when the drill is at the bottom of the well. If thishappens, the device must be pulled from the hole and quite possibly mayget stuck in the process. The above mentioned class of fixed offsetunder-reamers avoids this problem and provides an inherently simpler andstronger unit.

In reamers of the fixed type, the reaming cutters must necessarily bearranged assymmetrically with respect to the drill string axis in orderto pass through the bore hole to the point where the portion to beenlarged commences. As in the case of the retractable reamers, the fixedunder-reamers are usually combined with a pilot drill, which may beeither integral with the reamer body or a separate unit attachable tothe reamer by the conventional threaded coupling.

The radius of the pilot drill and the reamer offset radius are chosen sothat the combined tool will pass through the well casing by shifting thedrill pipe off cen- .ter with respect to the hole. Once the drillreaches the bottom of the hole, it tends to pilot itself and rotateconcentrically with the hole axis, thereby forcing the offset reamingcutter outward so as to enlarge the pilot hole to the desired finaldiameter.

Examples of such prior art offset under-reaming devices are shown in thefollowing US. Pats: No. 141,344, issued on July 29, 1873 to M.Gillespie; No. 1,065,294, issued on June 17, 1913 to A. G. Collins; No.1,587,266, issued on June 1, 1926 to J. A. Zublin; No. 2,953,354, issuedon Sept. 20, 1960 to E. B. Williams, Jr.; and No. 3,367,430, issued onFeb. 6, 1968 to D. S. Rowley. Another example is shown in Austrian Pat.No. 1430, issued on May 25, 1900 to B. von Vangel. Most of the bit andreamer combinations disclosed in these patents are very similar inprinciple, differing primarily only in the design of the bit and thereamer. In every case the reaming section extends radially outward ononly one side of the drill pipe axis so that the maximum cross sectionaldimension is less than the di ameter of the hole that the unit will reamwhen rotating concentrically about that axis.

The Gillespie device is unique in that it has no associated pilot bit;so it can be used only to under-ream a predrilled hole. To insure thatthe reamer rotates concentrically with the hole axis, Gillespie fillsthe predrilled hole with sand to the point where the enlargement is tocommence and then drops a tubular bearing member into the hole. Theoffset underreamer has a center pin at its lower end that fits looselyinside the tubular bearing, which thereby centers and guides therotating reamer without rotating itself. The Gillespie procedure andapparatus is impractical, however, for use in the deep drilled wells forwhich the present invention is primarily intended.

Collins discloses a twist type pilot drill below a blade type reamerthat acts as a boring tool for underreaming a hole. Zublin shows afishtail drill bit with an offset underreaming blade formed integrallywith and extending laterally from one edge of the fishtail blade. Rowleycombines in a single drill bit a lower diamond type pilot bit ofconventional design with a stepped, offset, diamond studded reamingsection directly above and contiguous with the pilot bit.

The pilot bit in each of these designs has cutting elements or edges onthe vertical sides of the drill which tend to gouge or counter-ream thepilot hole in response to unbalanced radial and tangential reactionforces acting on the reamer. As a result, the pilot drill and reamertend to rotate eccentrically and generate an outof-round hole.Furthermore, because the reamer is spaced axially above the drill facein the above three examples, it produces a force couple having a momentarm equal to the distance between the reamer and the drill face thattends to turn the pilot drill away from a straight line.

The van Vangel bit, which was a reference in the Rowley patent, has asquare-edged castellated pilot drill with fluted straight sides. Aneccentric, axially spaced offset reamer is of similar construction. Fromthe drawing it does not appear that the fluted sides are cutting sides,but the specification states only that the lower crown (of the pilotbit) serves to guide the upper eccentric crown in operation. Thecastellated design of the von Vangel bit is not suitable for present daydeep well drills, and the square-edged cutting face does not provide theself-piloting action of modern tapered-crown diamond bits. Thus the vonVangel bit will tend to drill away from a straight line, due to theforce couple between pilot bit and reamer, as described above.

Finally, the Williams patent discloses a diamond drill bit ofconventional design which has been modified to combine the drilling andunder-reaming functions by relieving a circumferential portion of thevertical surface of the drill extending over an arc of more than Thisreduces the maximum cross-sectional dimension of the drill and at thesame time creates in effect an offset reaming section from the portionof the vertical surface of the drill that remains.

The Williams bit tends to rotate about its original center because ofthe piloting action of its conically concave face against the bottom ofthe hole. Because this conical face is a cutting face, however, itspiloting action is counteracted to some extent by the unbalanced radialand tangential reaction forces acting on the reamer portion, in themanner described above, with a similar effect on the roundness andstraightness of the under-drilled hole.

To summarize, therefore, the prior art offset underdrilling devicesdescribed above all have a tendency to drill holes that are out-of-roundand off-direction because they depend on attached or integral pilot holedrilling bits to absorb the unbalanced radial and tangential forcesacting on the assymmetric reaming section.

SUMMARY OF THE INVENTION The stabilized well under-drilling apparatus ofthe present invention overcomes the disadvantages of the prior artunder-reaming bits by axially separating the drill bit and offset reamerby a substantial distance and positioning at least one stabilizing meansor guide member between the bit and the reamer. The guide member has aneffective diameter of substantially the same size as the hole drilled bythe pilot bit so that it snugly fits into the hole. An important featureof the stabilizing means is that it has no cutting elements on itsoutside diameter, although it may have flushmounted inserts of tungstencarbide or similar wearresistant material to prevent erosion of thebearing surfaces.

The guide member has two functions First, it stabilizes the pilot drillso that it drills the hole without deviating from a straight line.Secondly, it provides a substantial bearing surface against the wall ofthe pilot hole to assure that the offset reamer rotates concentricallywith the axis of the pilot hole. In this way, the final under-reamedportion of the well hole will be round and coaxial with the previouslydrilled and cased portion and will continue in a straight line from theend of the casing.

It is recognized that stabilizing devices for controlling drill bitdirection are well known, but their use for the functions of the presentinvention has never been suggested despite the long development historyof underdrilling devices and their attendant drawbacks. A pertinentexample is a convertible drilling tool disclosed in U.S. Pat. No.2,034,075, issued on Mar. 17, 1936 to .I. C. Wright. The Wright toolcomprises a shank having a drill bit attached at its lower end formaking a pilot bore and a stabilizing member mounted on the shank abovethe bit for guiding the bit during drilling of the pilot hole. Thestabilizing member is removable from the shank so that it may bereplaced by a bore enlarging cutter, which is then guided by the drillbit, not the stabilizer, to enlarge the predrilled pilot hole.

Specifically then, the apparatus of the present invention comprises astabilized offset reamer for enlarging a pilot hole of predetermineddiameter, the reamer having an elongated cylindrical body, a reamingsection extending outward from a portion of the circumference of theupper part of the body subtending an angle of less than 180, and a firststabilizing section below the reaming section, the stabilizing sectionhaving a smooth, non-cutting, bearing surface defined by a cylinderconcentric with the axis of the reamer body and having a diameterapproximately equal to the diameter of the pilot hole.

Although the stabilized reamer of the present invention can be usedalone to ream or under-ream a bore hole after predrilling the pilothole, the preferred embodiment further comprises a drill bit attached tothe lower end of the ream body and having a cutting head,

spaced from the lower end of the stabilizing section for simultaneouslydrilling the pilot hole as the reamer enlarges it.

An alternate embodiment of the invention further comprises a secondstabilizing section also having smooth, noncutting, bearing surfacesdefined by a cylinder concentric with and of approximately the samediameter as the first cylinder, the upper end of the second stabilizingsection being spaced from the lower end of the first stabilizingsection. The second stabilizing section can be integral with the reamerbody, or it can be a separate unit attachable to the lower end of thereamer body and having provision for attachment of a pilot bit at itslower end.

Accordingly, it is the object of this invention to provide a simple andsturdy device for under-reaming a bore hole so that the under-reamedportion is substantially round and coaxial with a previously drilledhole.

A further object of this invention is to provide means for guiding anoffset reamer so that the reaming element rotates concentrically withthe center line of a pilot hole.

A still further object of the invention is to provide a stabilizedunder-drilling device for simultaneously drilling a directionally guidedpilot hole and concentrically under-reaming the pilot hole to asubstantially round cross section of predetermined final diameter.

These and other objects, advantages, and features of the invention willbecome more apparent from the following description in conjunction withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 .is an elevation view in crosssection of a preferred embodiment of the stabilized drilling andunderreaming apparatus of the present invention in operative position ina bore hole.

FIG. 2 is a cross section of the reaming section of the apparatus ofFIG. 1.

4-3 is a cross section of the stabilizing section of the apparatus ofFIG. 1.

FIG. 4 is an elevation view in cross section of an alternate embodimentof the invention having two stabilizing sections between the drill bitand the reamer.

FIG. 5 is a cross section of the reaming section of the apparatus ofFIG. 4.

FIG. 6 is a cross section of the lower stabilizing section of FIG. 4.

FIG. 7 is an elevation view in cross section of another alternateembodiment of the invention.

FIG. 8 is a cross section of the reaming section of the apparatus ofFIG. 7.

FIG. 9 is a cross section of the upper stabilizing section of theapparatus of FIG. 7.

FIG. 10 is a cross section of the lower stabilizing section of theapparatus of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-3, arotary drill and reaming assembly 20 is shown in position for drillingand underreaming a well hole 10. Assembly 20 includes a pilot bit 21having a cutting head including a conical concave cutting face 22, aconvex crown portion 23 and cylindrical gauging sides 24 at the forwardend of the drill bit. Drill 21 is preferably a conventional type ofdiamond drill bit.

The upper end of drill bit 21 has an externally threaded portion 25which screws into a mating internally threaded portion 26 at the lowerend of a stabilized reaming means 27. The upper end 28 of reaming means27 is also externally threaded to screw into a mating internallythreaded end 29 of a conventional drill pipe. Reaming means 27 has acylindrical tubular body which includes a lower stabilizing section 30and an upper reaming section 31. Reaming section 31 comprises anintegral, radially offset portion having a lower face 32 taperingoutward from the upper end of stabilizing section 30 to join an uppercylindrical face 33 that is concentric with the drill pipe axis but hasa radius greater than the radii of the pilot drill 21 and of thestabilizing section 30. The offset reamer surfaces 32-33 extend over anarcuate distance substantially less than as shown in FIG. 2; thispermits the reamer to pass through a smaller hole (indicated bybrokenline circle 34) than the finished size of the under-reamed hole(shown by full-line circle 35). Since the diameter of circle 34 is afunction of thecircumferential extent of (or angle subtended by) upperreaming face 33 as well as of the difference between the radius of face33 and the radius of the pilot bit, it is desirable to make thiscircumferential extent as small as possible, consistent with permissibleloading pressures on the cutting elements, such as diamonds 36, embeddedin reaming faces 32 and 33. I

In order to lubricate and cool the drill and reamer and to flush awaythe rock cuttings, drilling fluid is pumped down an inner passageway 37through center of the drill pipe and connecting passages 38 and 39through the reamer and drill bodies, respectively, to the face of thedrill. The fluid then flows through circumferentially spaced waterways40 in the face and sides of the drill head, through the annular regionbetween the side of the pilot hole and the bodies of the drill bit andreaming means, through circumferentially spaced grooves 41 in thereaming surfaces 32 and 33, and finally upward between the wall of thereamed hole and the outside of the drill pipe for discharge at thesurface.

Referring to FIG. 3, stabilizing section 30 comprises a number ofcircumferentially spaced lands 42, together with a downward extension 43of reaming surface 32. The outer surfaces of lands 42 and extension 43are defined by a cylinder concentric with the axis of the pilot drilland the reamer body and having a diameter approximately equal to that ofthe pilot hole (indicated by full-line circle 44) but with sufficientclearance to permit rotation in the hole without binding.

- ing flushmounted inserts of hardened material 47, these inserts arenot necessary if the material of the lands is sufficiently hard andwear-resistant. The important feature is that stabilizer 30 does notperform any drilling or reaming function but is purely a pilotingbearing surface. This bearing surface, smoothly lubricated by the upwardflow of the drilling fluid through grooves 41 and passageways 46 betweenits lands, is thus able to withstand severe unbalanced radial forces andmoments acting on the offset reaming section 31 without cutting into thewall of the pilot hole. In this way, the stabilizing section insuresthat the reamed hole will be round and concentric with the pilot hole.At the same time, the section stabilizes pilot drill 21 to prevent anytendency to wander off the axis of the previously bored hole, eitherbecause of side forces from the reamer or uneven hardness of thematerial being drilled.

The embodiment of FIGS. 1-3 has only a single stabilizing meanspositioned adjacent to the lower end of the reamer. For even betterresults, it is preferred to add a second stabilizer between the firststabilizer and the pilot drill bit. Two embodiments of such anarrangement are shown in FIGS. I-6 and 7-10, respectively. Elements ofthese embodiments which are substantially the same as correspondingelements in the first embodiment are identified by the same referencenumerals.

Referring to FIGS. 4-6, the under-drilling assembly includes a pilotdrill 21, which may be identical to the drill in the first embodimentand a stabilized underreamiing member 50. Member 50 has an upper offsetreaming section 31 and a first stabilizing section 30, adjacent to thelower end of the reaming section, that are similar to the constructionin FIG. I. Below first stabilizing section 30 is a second stabilizingmeans 51 which is integral with member 50. As in the first embodiment,the drill bit is threadedly connected to the stabilizing and reamingmember, and that member, in turn, is threadedly connected to the lowerend of a drill pipe 29.

The offset radius of the reaming section in this second embodiment iscomparatively larger in relation to the radius of the pilot hole than isthe case in the apparatus of the first embodiment. The relation betweenthese radii is a matter of design choice within the limits imposed bythe forces on the drill and on the reamer.

Obviously, the smaller the drill diameter, the greater will be thepermissible offset radius of the reamer that will still allow theassembly to pass through a given size of well casing and, consequently,provide the largest possible diameter of the under-reamed hole. Thepilot drill should not be so small, however, that an undesirably largeshare of the drilling forces will be carried by the relatively smallsurface area of the reamer. Within this design limitation, the advantageof having two stabilizer sections is that a larger ratio of reamerradius to drill radius is possible without deflecting the drill orcausing the reamer to rotate non-concentrically.

Lower stabilizing means 51 is similar in design to the first stabilizingmeans, comprising three concentrically spaced lands 52 having inserts ofhardened material 53 flush-mounted with the outer surfaces 54 of thelands. Surfaces 54 define a cylinder having substantially the samediameter as the surfaces of the upper stabilizer. Passageways 55 betweenthe lands permit unobstructed flow-of the drilling fluid.

Although surfaces 54 are smooth and non-cutting, cutting elements 55 areshownimbedded in the inwardly tapered lower end of stabilizing section51. These cutting elements merely clean the pilot hole of minor roughareas that would otherwise tend to rapidly wear thebearing surfaces 54.

In the second embodiment, the reamer and upper stabilizing sections areintegral with the member carrying the lower stabilizing section. Thisdesign can result in a bulky and heavy unit which is both difficult tomanufacture and to handle in the field. Furthermore, excessive wear ofany one portion, such as the reaming section, requires that the wholeunit be sent back to the factory for reworking.

To avoid this problem, the embodiment of FIGS. 8 and 10 shows anassembly in which the pilot bit, the reaming and upper stabilizingsections, and the lower stabilizing means are in three separate units,threaded together. As in the previous embodiments, the drill bit can bea conventional diamond drill 21. Second stabilizing means 51 is similarin construction to the lower stabilizer of FIG. 4 except that it hasfive lands as shown in FIG. 10 instead of the three lands of the lowerstabilizer shown in FIG. 6. The number of lands is merely a matter ofdesign, however, depending on the desired ratio of bearing surface todrilling fluid channel area.

The design of upper stabilizing section 30 differs somewhat, however,from that of the other two embodiments by eliminating the bearingsurface portion 43 in the region underlying the offset reaming section31, as shown in FIG. 9. This provides increased clearance for the flowof lubricating and cooling drilling fluid to the cutting face of thereamer. At the same time the three broad lands 42 opposite the reamerprovide adequate bearing surfaces to withstand the unbalanced side loadscaused by the reactive radial and tangential forces acting on thereamer.

Although the above embodiments illustrate some variations andmodifications possible within the frame work of the invention, it willbe apparent that other variations may be made without departing from itsspirit and scope.

What is claimed is:

l. A rotary under-reaming device for enlarging a pilot hole vofpredetermined radius drilled in the earth, the device comprising:

an elongated cylindrical body having a radius no greater than thepredetermined radius of the pilot hole and a threaded upper end forattachment to a drill pipe;

an asymmetrical reaming section extending outwardly from the upper partof the body about only an arcuate portion of the circumference thereofto a radius greater than the radius of the pilot hole, the arcuateportion subtending an angle of less than 180 and the remainder of saidcircumference having a non-reaming face, and

a first non-reaming stabilizing section located on the body below thereaming section, the stabilizing section having a smooth, non-cutting,bearing surface defined by a first cylinder concentric with thelongitudinal axis of the cylindrical body and having a radiusapproximately equal to the predetermined radius of the pilot hole buthaving sufficient clearance to rotate in the pilot hole without binding,whereby the reaming section is caused to rotate concentrically with theaxis of the pilot hole.

2. The under-reaming device of claim 1 further comprising:

a drill bit attached to the lower end of the reamer body and having acutting head axially spaced from the lower end of the first stabilizingsection for simultaneously drilling the pilot hole as the reamingsection enlarges it.

3. The under-reaming device of claim 1 further comprising:

a second non-reaming stabilizing section located below and axiallyspaced from the first stabilizing section, the second stabilizingsection having smooth, non-cutting bearing surfaces defined by a secondcylinder coaxial with and having the same diameter as the firstcylinder.

4. The under-reaming device of claim 3 further comprising:

a drill bit located below the second stabilizing section and having acutting head axially spaced from the lower end of the second stabilizingsection for drilling the bottom of the pilot hole as the reaming sectionenlarges its sides.

5. The under-reaming device of claim 3 wherein:

the first stabilizing section comprises a plurality of axially extendingfirst lands circumferentially spaced about the cylindrical body in theregion complementary to the circumferential region subtended by thereaming section, the outer surfaces of the first lands comprisingwear-resistant, noncutting, bearing surfaces defined by the firstcylinder; and

the second stabilizing section comprises a plurality of axiallyextending second lands symmetrically spaced about the cylindrical body,the outer surfaces of the second lands comprising wearresistant,non-cutting, bearing surfaces defined by the second cylinder, wherebythe bearing surfaces of the first and second lands cooperate to subtenda major portion of the circumference of the pilot hole.

6. The under-reaming device of claim 5 further comprising a centralpassageway through the cylindrical body for conducting drilling fluidunder pressure downward through the bottom of the body and then upwardthrough the spaces between the lands of the second and first stabilizingsections, and wherein the reaming surface contains circumferentiallyspaced grooves to permit a portion of the fluid flowing upward from thefirst stabilizing section to lubricate and cool the reaming surface.

7. The under-reaming device of claim 1 wherein the reaming sectioncomprises a cutting surface defined by a circumferential portion of acylinder concentric with the axis of the reamer body and having a radiuslarger than the predetermined radius of the pilot hole.

8. The under-reaming device of claim 7 wherein the first stabilizingsection comprises a plurality of axially extending landscircumferentially spaced about the cylindrical body in the regioncomplementary to the circumferential region subtended by the reamingsection.

9. A rotary under-reaming device for increasing the radius of a pilothole of predetermined radius drilled in the earth, the devicecomprising:

an elongated cylindrical body having a threaded upper end adapted forattachment to a drill stem and a threaded lower end adapted forattachment to a stabilizing element and an asymmetrical reaming sectionextending outwardly from a first arcuate circumferential portion of thecylindrical body, the reaming section subtending an angle of less thanand the remaining circumference of the cylindrical body having anon-reaming surface, the reaming section including a lower cuttingsurface extending outwardly from the first portion of the circumferenceof the cylindrical body to a radius greater than the radius of the pilothole and an upper surface defined by a portion of a cylinder having aradius substantially equal to the maximum radius of the cutting surface,the lower cutting and upper reaming surfaces having rows of embeddedcutting elements separated by upwardly directed passages for drillingfluid to cool and lubricate the cutting elements.

10. The under-reaming device of claim 9 wherein a second circumferentialportion of the reamer body above the lowest extent of the reamer cuttingsurface in the region not subtended by the reaming section has a radiusapproximately equal to the radius of the pilot hole.

Ill. The under-drilling device of claim 10 wherein the secondcircumferential portion comprises the outer surfaces of a plurality ofspaced lands separated by upwardly directed passageways for drillingfluid.

. UNITED STATES PATENT OFFICE O CERTIFICATE OF CORRECTION Patent No.3,851,719 Dated December 3, 1974 Inventor) Charles 1', Thompson andBobby J. Thompson It: is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 7, line 45, change "FIGS. 1-6" to -FIGS. 46-.'

Column 7, line 52, change "reamiing" to reaming--. Column 10, line 42,change "uper surface" to -upper reaming -surface--.

Signed sealed this llth daj of Februaryll975.

(SEAL) Attest: c

' c MARSHALL DANN RUTH MASON/ Commissioner of Patents Attestlng Off cerand Trademarks FORM PO-IOSO (10-69) USCOMWDC 63 6. I '2 [L5, GOVERNMENTPRINTlNG OFFICE I," 36'33,

1. A rotary under-reaming device for enlarging a pilot hole ofpredetermined radius drilled in the earth, the device comprising: anelongated cylindrical body having a radius no greater than thepredetermined radius of the pilot hole and a threaded upper end forattachment to a drill pipe; an asymmetrical reaming section extendingoutwardly from the upper part of the body about only an arcuate portionof the circumference thereof to a radius greater than the radius of thepilot hole, the arcuate portion subtending an angle of less than 180*and the remainder of said circumference having a non-reaming face, and afirst non-reaming stabilizing section located on the body below thereaming section, the stabilizing section having a smooth, non-cutting,bearing surface defined by a first cylinder concentric with thelongitudinal axis of the cylindrical body and having a radiusapproximately equal to the predetermined radius of the pilot hole buthaving sufficient clearance to rotate in the pilot hole without binding,whereby the reaming section is caused to rotate concentrically with theaxis of the pilot hole.
 2. The under-reaming device of claim 1 furthercomprising: a drill bit attached to the lower end of the reamer body andhaving a cutting head axially spaced from the lower end of the firststabilizing section for simultaneously drilling the pilot hole as thereaming section enlarges it.
 3. The under-reaming device of claim 1further comprising: a second non-reaming stabilizing section locatedbelow and axially spaced from the first stabilizing section, the secondstabilizing section having smooth, non-cutting bearing surfaces definedby a second cylinder coaxial with and having the same diameter as thefirst cylinder.
 4. The under-reaming device of claim 3 furthercomprising: a drill bit located below the second stabilizing section andhaving a cutting head axially spaced from the lower end of the secondstabilizing section for drilling the bottom of the pilot hole as thereaming section enlarges its sides.
 5. The under-reaming device of claim3 wherein: the first stabilizing section comprises a plurality ofaxially extending first lands circumferentially spaced about thecylindrical body in the region complementary to the circumferentialregion subtended by the reaming section, the outer surfaces of the firstlands comprising wear-resistant, non-cutting, bearing surfaces definedby the first cylinder; and the second stabilizing section comprises aplurality of axially extending second lands symmetrically spaced aboutthe cylindrical body, the outer surfaces of the second lands comprisingwear-resistant, non-cutting, bearing surfaces defined by the secondcylinder, whereby the bearing surfaces of the first and second landscooperate to subtend a major portion of the circumference of the pilothole.
 6. The under-reaming device of claim 5 further comprising acentraL passageway through the cylindrical body for conducting drillingfluid under pressure downward through the bottom of the body and thenupward through the spaces between the lands of the second and firststabilizing sections, and wherein the reaming surface containscircumferentially spaced grooves to permit a portion of the fluidflowing upward from the first stabilizing section to lubricate and coolthe reaming surface.
 7. The under-reaming device of claim 1 wherein thereaming section comprises a cutting surface defined by a circumferentialportion of a cylinder concentric with the axis of the reamer body andhaving a radius larger than the predetermined radius of the pilot hole.8. The under-reaming device of claim 7 wherein the first stabilizingsection comprises a plurality of axially extending landscircumferentially spaced about the cylindrical body in the regioncomplementary to the circumferential region subtended by the reamingsection.
 9. A rotary under-reaming device for increasing the radius of apilot hole of predetermined radius drilled in the earth, the devicecomprising: an elongated cylindrical body having a threaded upper endadapted for attachment to a drill stem and a threaded lower end adaptedfor attachment to a stabilizing element and an asymmetrical reamingsection extending outwardly from a first arcuate circumferential portionof the cylindrical body, the reaming section subtending an angle of lessthan 180* and the remaining circumference of the cylindrical body havinga non-reaming surface, the reaming section including a lower cuttingsurface extending outwardly from the first portion of the circumferenceof the cylindrical body to a radius greater than the radius of the pilothole and an upper surface defined by a portion of a cylinder having aradius substantially equal to the maximum radius of the cutting surface,the lower cutting and upper reaming surfaces having rows of embeddedcutting elements separated by upwardly directed passages for drillingfluid to cool and lubricate the cutting elements.
 10. The under-reamingdevice of claim 9 wherein a second circumferential portion of the reamerbody above the lowest extent of the reamer cutting surface in the regionnot subtended by the reaming section has a radius approximately equal tothe radius of the pilot hole.
 11. The under-drilling device of claim 10wherein the second circumferential portion comprises the outer surfacesof a plurality of spaced lands separated by upwardly directedpassageways for drilling fluid.